Diesel fuel combustion processes and newer Otto-cycle combustion processes (for example so-called Gasoline Homogeneous Charge Compression Ignition (HCCI) processes or Controlled Auto Ignition (CAI) processes) differ from conventional applied-ignition processes in that the ignition of the fuel is triggered not by applied ignition, for example by means of an ignition plug, but by the mixing of a fuel and re-introduced exhaust gas at an elevated temperature. In this case, relatively homogeneous mixture formation occurs before the ignition, which normally leads to a multiplicity of exothermic centers in the combustion chamber. The actual combustion of the fuel thus takes place relatively uniformly and very rapidly, which in turn leads to reduced fuel consumption and to reduced pollutant emissions.
The temporal position of an auto-initiated fuel combustion is often specified in terms of a crankshaft angle. It is preferable for the temporal position of a specific energy conversion of the combustion to be used for this purpose. For example, the center of combustion may be specified as MFB50 (Mass Fraction Burnt 50%) or HR50 (Heat Release 50%).
In the case of an auto-ignition internal combustion engine, however, there is no direct trigger for the initiation of the combustion, such as for example an ignition spark of an ignition plug. For this reason, relatively large, cylinder-specific differences may arise with regard to the progression of the combustion. This also applies with regard to combustion noise, which contributes to undesired acoustic emissions of an internal combustion engine.
For the adjustment, and in particular for the reduction, of combustion noise, it is known for combustion-relevant parameters relating to the fuel path and the air path to be varied.
DE 10 2008 000 552 A1 describes a method for the operation of an auto-ignition internal combustion engine, in which the following steps are performed: (a) pre-definition of a setpoint combustion position and of a setpoint combustion noise feature; (b) operation of a cylinder of the internal combustion engine for at least one cycle, maintaining a predefined first injector and/or air valve actuation variable and a predefined second injector and/or air valve actuation variable; (c) ascertainment of an actual combustion position and an actual combustion noise feature of the cylinder; (d) comparison of the actual combustion position with the setpoint combustion position and, if the actual combustion position deviates from the setpoint combustion position, re-determination of the first injector and/or air valve actuation variable; and (e) comparison of the actual combustion noise feature with the setpoint combustion noise feature and, if the actual combustion noise feature deviates from the setpoint combustion noise feature, re-determination of the second injector and/or air valve actuation variable. In this case, as an actual combustion noise feature, use may be made of a maximum pressure gradient in the cylinder, because the maximum pressure gradient of the combustion process correlates with the combustion noise. The maximum pressure gradient can be determined in a simple manner by means of a cylinder pressure acquisition system.